1. Related Application
The present application claims priority to Japanese Patent Application Number 2007-182818, filed Jul. 12, 2007, the entirety of which is hereby incorporated by reference
2. Field of the Invention
The present invention relates to a position sensing device and method for detecting a current position of a vehicle, and more specifically to a position sensing device and method capable of increasing the accuracy of a dead reckoning position.
3. Description of the Related Art
An in-vehicle navigating system employs both dead reckoning with a dead reckoning sensor and GPS (global positioning system) navigation with a GPS receiver.
Dead reckoning is a technique of detecting a vehicles position, orientation, and speed using an output of an acceleration sensor for detecting an acceleration of a vehicle, a relative direction sensor (gyro, etc.) for detecting a change in vehicle direction, and a distance sensor (speed sensor, etc.) for detecting a vehicle speed (distance over time). However, output data (position, orientation, speed vehicle, etc.) of the dead reckoning sensor includes an error in sensor measurement, so some error will occur. In particular, position and orientation are calculated by integrating an output value of the sensor, so an error is cumulatively increased. On the other hand, the GPS receiver can determine absolute values of a vehicle's position, orientation, and speed within the maximum positional error of about 30 m. Thus, if the output dead-reckoning position is adjusted to a position output from the GPS receiver upon receiving a GPS signal, the cumulative error can be corrected. For example, if a positional difference between a position determined with a GPS receiver and a dead reckoning position of a target vehicle to a position on any road on a road map based on any known map matching technique is larger than a predetermined value, the position on the road map is adjusted to the position determined with the GPS receiver.
The dead-reckoning position can be corrected in accordance with an output value of the OPS receiver as described above. However, dead reckoning has a problem that an error in dead reckoning cumulatively increases due to an error in sensor output value and sensor mounting angle when no GPS data is received, which deteriorates an output accuracy. In particular, a GPS signal cannot reach the navigation system in a vehicle in a multilevel parking structure or basement car park, and a positional error of about 100 m or less occurs. In addition, the navigation system often receives reflected GPS signals in an inner-city area. If multipath interference occurs, this interference involves a positional error of about 300 m or less.
Based on the above, a method of correcting an error in output sensor value to determine a current position has been proposed. A method disclosed in Japanese Unexamined Patent Application Publication No. 2001-337150 calculates an offset error, a distance factor error, an absolute orientation error, and an absolute position error through a Kalman filter process based on information about a vehicle's position, orientation, and speed measured through dead reckoning and information about the vehicle's position, orientation, and speed output from a GPS receiver, and corrects the errors.
To apply an output value of a GPS receiver (positional data and speed data in three directions, latitude, longitude, and height) to the Kalman filter process, it is necessary to determine an error index of each component of the output data of the GPS receiver (see Japanese Unexamined Patent Application Publication No. 2001-337150). However, the GPS receiver can only output an index of error with respect to a horizontal position. Thus, among the output data of the GPS receiver, data without an error index cannot be used for the Kalman filter process. As a result, the necessary error indexes are determined through simple calculation, but an accuracy of a position estimated with the thus-determined indexes is much lower than that estimated with correct indexes. In addition, an accuracy of output data of a conventional GPS receiver decreases due to an influence of multipath interference. In this case, an error index also becomes incorrect (an error should be large but a small value is output). In such cases, if the GPS output data is applied to the Kalman filter process, a position estimation accuracy considerably decreases.
To solve the above problem, a GPS receiver (available from u-blox) capable of outputting error indexes for all GPS output components is used. However, such a GPS receiver is costly and cannot be used. Here, a GPS navigation system that selectively uses GPS output data including fewer errors to increase a positioning accuracy is proposed (see Japanese Unexamined Patent Application Publication No. 8-334338). However, this system does not control the Kalman filter process based on an error value.